Your GamerScore (G) is the accumulation of points which are earned by unlocking achievements. GamerScore is a way to track gaming progress and give you bragging rights with your friends.
You can see your GamerScore in your Xbox Profile via the Xbox Console Companion App (Windows) or the Xbox App (mobile).
To increase your GamerScore, unlock more achievements to earn more points. You can see available achievements for each Microsoft Casual Game in Xbox Console Companion App (Windows), the Xbox App (mobile), or in-game. For steps on where to find achievements, click here.
And for more information about achievements, click here.